Reducing nox formation by combustion

ABSTRACT

Apparatus for mixing recirculated flue gases with combustion air delivered to a furnace to reduce the formation of NOx caused by the combustion of fuel. The recirculated gas is mixed with combustion air prior to delivery of the mixture to the furnace so that the mixture is substantially uniform when delivered to the combustion chamber.

United States Patent 1 1 Rudd et al. Dec. 25, 1973 REDUCING NOXFORMATION BY 2,688,360 9/1954 Haynes et al. 431 115 COMBUSTION 2,310,0963/1967 DeLivois 431/115 Inventors: Alexander H. Rudd, Akron; John H.

Kidwell, Alliance; Thomas J. Murray, Wadsworth, all of Ohio Assignee:The Babcock & Wilcox Company,

New York, NY.

Filed: Mar. 24, 1972 Appl. No.: 237,850

11.8. Cl. 431/115 Int. Cl. F23l 7/00 Field of Search 431/115, 116, 9,

References Cited UNITED STATES PATENTS 9/1964 Wuetig 431/115 X PrimaryExaminer-Edward G. Favors AttorneyJ. Maguire [5 7 ABSTRACT Apparatus formixing recirculated flue gases with combustion air delivered to afurnace to reduce the fom'iation of NO, caused by the combustion offuel. The recirculated gas is mixed with combustion air prior todelivery of the mixture to the furnace so that the mixture issubstantially uniform when delivered to the combustion chamber.

8 Claims, 3 Drawing Figures PATENTEUUECZS I975 SHEET 1 BF 2 FIG.

REDUCING NOX FORMATION BY COMBUSTION The present invention relates toapparatus for burning fuel with a reduction in the quantity of nitrogenoxides formed during the combustion of the fuel. Particularly theinvention relates to a system for controlling the combustion of the fuelso as to minimize the formation of nitrogen oxides during combustion.

Nitric oxide (NO) and nitrogen dioxide (N are often contained in theexhaust gases from combustion processes. Their presence is due toformation of NO during combustion and subsequent oxidation of a smallpart of the NO to NO by residual oxidation in the products of combustionpassing through a furnace chamber. After release to the atmosphere,oxidation of NO continues, and the resulting N0 is an air pollutant.Nitrogen dioxide is best known for its participation in smog formation,but also exhibits some deleterious effects of its own. Since NO is, froman air pollution point of view, synonymous with N0 the two are oftencombined and referred to as NO,,.

Not all combustion processes operate at sufficiently high temperature toproduce significant quantities of N0 The major producers are theautomobile, and industrial and utility boilers. Reduction of NOemissions from utility boilers on the West Coast in the late 1950s isone of the earliest examples of air pollution control. This was achievedprimarily by a modification of the combustion process known as two-stagecombustion as disclosed in US. Pat No. 3,048,131. More recently N0pollution has been shown to be a general nuisance in many parts of thecountry, and work has begun in both the automobile industry andcontinued in the boiler industry to reduce N0 emmissions to theatmosphere.

The amount of NO, formation in a boiler furnace is determined by thetemperature and composition history of the combustion gas during andafter combustion. This is related in turn to fuel supply conditions andother operating and design factors of the boiler. Some of the primaryfactors found to be of importance to NO, production relate to the rateof fuel supply and the values of oxygen available during and immediatelyafter combustion of the fuel (i.e., excess air used in the combustionprocess), the configuration and rate of cooling in the combustionchamber, and the temperature of the combustion air delivered to thefurnace and combined with the fuel during the combustion process. All ofsuch factors affect the generation of NO, during fuel combustion bydirectly or indirectly influencing temperatures in the combustionchamber.

l-leretofore, reduction of NO, production in existing large industrialand utility boiler units has been attempted by the injection ofrecirculated flue gases into the combustion chamber. This has not alwaysbeen successful, since it is important the recirculated flue gas be wellmixed with the combustion air so that each of the multiple burners insuch an installation receives its proportionate share of oxygen for fuelcombustion purposes. The burner or burners getting the least oxygen(because of faulty mixing or gas distribution) would either smoke orhave unstable and incomplete combustion of the fuel thereby limiting theamount of gas recirculation and the degree of NO, reduction.

According to the present invention it has been found that recirculatedgases can be intimately and thoroughly mixed with combustion airupstream of the point of fuel introduction into the boiler combustionchamber. In the usual boiler unit some device is provided to measure thecombustion air flow to the combustion chamber and in many installationsthe device may take the form of a restriction in the air supply ductsuch as a venturi or a venture-like tube, or one or more air foils.Advantageously in the present invention the recirculated gases, for NO,reduction purposes, is added downstream of the air flow restriction (inan airflow sense) to minimize the pressure and thus the power requiredby the recirculated gas fan, and to insure intimate mixing of therecirculated gases with the combustion air. Of the drawings:

FIG. 1 is a schematic elevation of a conventional power boilerincorporating the present invention;

FIG. 2 is an enlarged view, in section, of a portion of the apparatusshown in FIG. 1; an

FIG. 3 is an isometric view, partly in section, of the apparatus shownin FIG. 2.

In the illustrated embodiment of the invention depicted in FIG. 1 aconventional power generation boiler is shown. In the particulararrangement the furnace and boiler setting 10 includes steam generatingtubes 11 so arranged that a combustion chamber 12 is positioned in thelower portion of the setting. The gases of combustion produced in thecombustion chamber pass upwardly over vapor heating surfaces 13, turnand then pass downwardly over additional heat exchange surfaces 14before discharge through a connecting duct 15 to a conventional airheater 16. The heat exchange surfaces may include superheater elements,reheater elements, and economizer surfaces. The surfaces are intended toheat the fluids passing therethrough and to reduce the temperature ofthe flue gas discharging from the setting 10 through the duct 15 to theair heater 16.

The air heater illustrated is of the regenerative type where relativelylow level heat from the flue gas is transferred to combustion airentering through a duct 17 which is thereafter passed through a duct 18to a windbox 20 and burner ports 21 serving the chamber 12. Ordinarily,in units of the type described the combustion air flow to the burnerports 21 is regulated to be in proper flow relationship to the fuel alsobeing supplied through bumers positioned in the ports 21 and thus to thecombustion chamber 12. To this end, in many installations streamlinedair foils 22 are installed in the air duct 18 so as to measure the flowof air therethrough and to provide a measurement for use in the fuel-aircontrol of the unit. The air foils illustrated in FIG. 1 are shown ingreater detail in FIGS. 2 and 3 and.

trated each row 23 and 24 of burners and burner ports in the lowermostportion of the chamber 12 contain 4 horizontally spaced burners whilethe uppermost row 25 contains 2 burners. It will be appreciated thatgreater or lesser numbers of ports and burners may be utilized, and anyfuel could be used.

In the installation illustrated in FIG. 1, a row of ports 26 ispositioned above the uppermost row 25 of bumers. Specifically, in theembodiment shown, there are 4 ports in each row 26 and the spacing in avertical direction between the uppermost row 25 of the burner ports andthe ports 26 is equal to or greater than the vertical spacing betweenthe rows 23, 24 and 25 of fuel burner ports.

In the embodiment shown, gas recirculation is utilized for vaportemperature control purposes. Such gases may be introduced into thefurnace at any of various positions. As shown, gases are withdrawn fromthe duct and passed through connecting ducts 27, through fan 35 and duct40, to gas plenum chambers 28 formed beneath the inclined tubes 30defining the bottom of the chamber, and which receive the recirculatedgas and pass this gas upwardly through an opening 31 in the bottom ofthe furnace to mix with the products of combustion in passing over theheat exchange surfaces 13 and 14 in the upper portion of the setting 10.As is well known in the art, recirculated flue gases when used for vaportemperature control purposed increase with the decrease in the rate offuel firing. The intent of this type of operation is to regulate gasmass flow over the heat exchange surfaces 13 and 14. This regulates heatexchange to the vapor heating surfaces during low load operations.

It has recently been found that the addition of recirculated gases withthe combustion air admitted to the burners with the fuel aids inreducing the nitrogen oxide produced during the combustion process. Oneof the difficulties heretofore encountered has been related to a propermixing of the recirculated gases with the combustion air as the twogases are injected into the combustion chamber. Ordinarily, the use ofrecirculated gases to the furnace chamber for nitrogen oxide controlpurposes has required a maximum flow of such gases to the furnace duringperiods of maximum fuel combustion. That is, of course, the reverse ofthe normal use of recirculated gases for vapor temperature controlpurposes.

In accordance with the present invention, the recirculated gases removedfrom the flue gas duct 15 are mixed with the combustion air before themixed gases are delivered to the windboxes of the furnace setting. Inthe usual installation the air flow duct through which combustion air issupplied to the windbox and thereafter into the furnace is provided withsome device for measuring the flow of air through the duct. This devicecommonly is a restriction, such as an orifice of the plate type, one ormore venturis or venturilike tubes, or in many installations one or moreair foils arranged in parallel to permit air flow measurements bydifferential pressures. The restrictive devices are selected to minimizeloss of pressure in the air flowing through the duct for fan powereconomy while also providing sufficient differential pressure values foradequate flow measurement. With such restrictions in the air flow ductwe have found that the recirculated flue gases used for NO, controlpurposes can advantageously be admitted to the air stream downstream ofthe restrictor for air and gas mixing purposes while minimizing fanpressure and power requirements in the recirculated flue gas ductopening into the air flow duct.

In the embodiment shown the air foils 22 are utilized for mixingpurposes to ensure a substantially uniform and adequate mingling ofcombustion air and recirculated gases before they enter the combustionchamber l2.

Referring to FIGS. 2 and 3 it will be noted the air foils 22 asordinarily installed for air flow measurement purposes are positioned inthe duct 18 in single or multiple layers, depending upon the volume ofair flow, with the large end 32 of the air foil facing upstream insofaras air flow direction is concerned. In the usual air flow measurementprocedure, a pressure measurement is obtained by positioning a tap 33 onthe upstream facing edge of the air foil. One or more taps 34 arepositioned at right angles to the air flow direction adjacent thethickest portion of the air foil. The differential pressure measurementsbetween the taps 33 and 34 will be indicative of the rate of air flowthrough the duct 18. This differential pressure measurement whenproperly calibrated can be utilized to measure the rate of air flowthrough the duct, and has also been utilized in air flowsteam flowcontrol arrangements to coordinate air and fuel delivery to thecombustion chamber 12.

Recirculated gas is passed from the duct 27 through a recirculated gasfan 35 and a dampered branch duct 36 with the recirculated gasesdelivered to the interior of the air foils 22. Ordinarily, such gasesare delivered to both ends of the hollow air foil particularly when thelength of the air foil is substantiaLas is usually encountered in alarge capacity boiler unit.

sliown pa rticularly in FIG. 3 ,an din accordance with the presentinvention, openings 37 are provided in the air foil to interconnect theinterior of the air foil with the flow path of the combustion air streamalong the exterior surface of the foil. These openings are spaced alongthe longitudinal length of the air foil at positions down stream of themaximum thickness of the air foil shape. With the openings so positionedthe aspiration effect of combustion air flow over the foils will tend todraw recirculated gas from the hollow foils into the stream of airpassing through the air duct 18. Due to such flow it has been found thatthe recirculated gases intimately mix with the combustion air so that anearly equally mixed combination of the two gases may be delivered tothe wind box 20 and through each of the ports 23, 24 and 25 with thefuel to the combustion chamber 12.

In the arrangement shown in FIG. 1, the concept of US. Pat. No.3,048,131 may also be used with the present invention by passing acontrolled portion of the mixture of combustion air and recirculatedflue gases through the ports 26. Under such circumstances fuel will notusually be introduced through the ports 26 with the mixture of air andgas. Such a combination will have a further effect in reducing theformation of NO, by fuel combustion.

" G obd mixing of recirc ulated flue garish the inter ior of each foilinto the combustionair passing over the exterior of the foil can beaccomplished by forming the openings 37 as slots having a length four tosix times the width and having an inter port spacing of from three toeight times the width of each slot. In addition, the upstream edge 01the slot shbuld be at least 13 of the widest part of the foil. Underthese conditions it has been found the mixing of air and gas issubstantially complete.

We claim:

fuel combustion which comprises walls defining a combustion chamber,means defining a plurality of burner ports in a wall of the combustionchamber, burner means positioned to inject fuel into the combustionchamber through each of the burner ports, a windbox enclosing the burnerports for the delivery of combustion air through the burner ports withthe fuel injected therethrough for combustion of fuel in the combustionchamber, air flow duct means connecting the windbox with a source ofpreheated combusted air, means including a restrictor for measuring anddampers and a fan for regulating the flow of combustion air through theduct means to the windbox in coordinated response to the rate of fuelflow through the ports, flue gas duct means interconnecting the gasdischarge end of the combustion chamber with the air flow duct meansadjacent the restrictor, said flue gas duct means including dampers anda recirculated gas fan for passing recirculated flue gas to the air flowduct for mixing flue gas with the preheated combustion air and deliveredthe mixture gases through siad burner ports to reduce the percentage ofoxygen present in the gas mass delivered to the combustion chamber andto thereby reduce the formation of NO, by fuel combustion.

2. Apparatus according to claim 1 wherein the recirculated flue gas ductopens into said air flow duct at a position downstream, in an air flowsense, of the flow restrictor.

3. Apparatus according to claim 2 wherein said air flow restrictorconsists of at least one foil extending across the duct.

4. Apparatus according to claim 3 wherein the foil is hollow and therecirculated flue gas duct opens into the hollow foil with atransversely spaced series of gas discharge openings positioned todischarge through the foil into the air stream passing over the foildownstream of the maximum transverse dimension of the foil.

5. Apparatus according to claim 4 wherein the recirculated gas ductopens into the opposite ends of said foil.

6. Apparatus according to claim 2 wherein the air flow restrictorconsists of a venturi-like tube.

7. Apparatus according to claim 6 wherein the recirculated flue gasopens into the air stream passing through the venturi-like tube aroundand downstream in an air flow sense of the minimum cross-sectional areaof the venturi-like tube.

8. Apparatus according to claim 1 wherein ports are provided in saidwall spaced from the burner ports and within the windbox for thecontrolled introduction of mixed combustion air and recirculated gasesonly into combustion chamber.

2. Apparatus according to claim 1 wherein the recirculated flue gas ductopens into said air flow duct at a position downstream, in an air flowsense, of the flow restrictor.
 3. Apparatus according to claim 2 whereinsaid air flow restrictor consists of at least one foil extending acrossthe duct.
 4. Apparatus according to claim 3 wherein the foil is hollowand the recirculated flue gas duct opens into the hollow foil with atransversely spaced series of gas discharge openings positioned todischarge through the foil into the air stream passing over the foildownstream of the maximum transverse dimension of the foil.
 5. Apparatusaccording to claim 4 wherein the recirculated gas duct opens into theopposite ends of said foil.
 6. Apparatus according to claim 2 whereinthe air flow restrictor consists of a venturi-like tube.
 7. Apparatusaccording to claim 6 wherein the recirculated flue gas opens into theair stream passing through the venturi-like tube around and downstreamin an air flow sense of the minimum cross-sectional area of theventuri-like tube.
 8. Apparatus according to claim 1 wherein ports areprovided in said wall spaced from the burner ports and within thewindbox for the controlled introduction of mixed combustion air andrecirculated gases only into combustion chamber.